1. Field of the Invention
The invention relates to moisture-curing polyurethane prepolymers containing free isocyanate groups, to a process for preparing them and to their use as binders in paints and coatings.
2. Description of the Prior Art
Moisture-curing prepolymers have long been known as a constituent of one-component coating materials (e.g. H. Kittel, Lehrbuch der Lacke und Beschichtungen, 2, S Hirzel Verlag Stuttgart, Leipzig, 1998). These systems are composed of polyurethane prepolymers containing terminal free isocyanate groups (NCO groups). Following application, under the influence of atmospheric moisture, crosslinking takes place with formation of urea groups.
For rapid drying, prepolymers based on aromatic isocyanates, particularly 2,4- and possibly 2,6-diisocyanatotoluene (TDI), are most suitable.
Particularly the polyisocyanates based on the TDI grades referred to and containing isocyanurate groups constitute valuable constituents for the production of quick-drying polyurethane coatings for the wood and furniture sectors.
The reason for the rapid drying of the TDI isocyanurates is the high degree of branching. This leads to a rapid increase in the glass transition temperature in the course of crosslinking with the formation of urea groups. As a result of the high degree of crosslinking, however, rapid-drying systems of this kind are often brittle and films formed from such coating materials do not exhibit the requisite elasticity. Particularly in the case of sharp temperature changes it is thus possible for cracks to form in the coating. This is especially true of coatings on substrates which are subject to sharp fluctuations in volume, such as wood.
For flexibilization (suitability is possessed by), are suitable for example, high molecular weight polyethers for synthesizing the prepolymer. Flexibilization generally makes the polymers soft such that the drying rate is markedly retarded. In practice this means that such moisture-curing coatings based on polyurethane prepolymers dry much more slowly than, for example, the widespread combined nitrocellulose coating materials (NC lacquers).
Moreover, the compatibility of long-chain polyethers with TDI isocyanurates is limited, occasionally leading, even with smaller amounts of polyether, to products which are cloudy or lack storage stability.
It was an object of the present invention, accordingly, to provide polyurethane prepolymers allowing the preparation of coating materials and coatings which have drying rates comparable with those of NC lacquers and which at the same time achieve the requisite elasticity and the conventionally high resistance level of polyurethane coating materials.